Field of the Invention
The present invention relates to an image reading apparatus and an image forming system.
Description of Related Art
Heretofore, an image forming system consisting of an electrophotographic image forming apparatus and an image reading apparatus has been used. The image forming apparatus is capable of successively printing images on sheets. The image reading apparatus is provided with a scanner and a colorimeter.
A scanner can be purchased at a lower price and read an image over a wide area at a higher reading speed than a colorimeter. On the other hand, while color information can be read only from a limited area at a lower reading speed than a scanner, a colorimeter can read color information with a high degree of accuracy.
An image reading apparatus performs various types of processing by taking advantages of the features of a scanner and a colorimeter respectively. For example, an image reading apparatus reads an image printed by an image forming apparatus with a scanner, and feeds back read information to the image forming apparatus in order to correct the color, position, magnification factor and the like of the image. Specifically, an image reading apparatus adjusts a color information read by a scanner based on a color information read by a colorimeter.
Also, an image reading apparatus confirms the basic performance of a colorimeter. For example, unevenness in the light intensity of a colorimeter is corrected as a process performed for each colorimetric measurement. The unevenness in the light intensity of a colorimeter is corrected by using a white tile as a reference plate and measuring the light reflected from the white tile. Also, as a process performed during maintenance, it is roughly confirmed if wavelengths are shifted. This process makes use of a green tile as a reference plate which is colored green in correspondence with an intermediate wavelength of the visible light range, and is performed by confirming wavelength shift amounts over the entirety of the visible light range.
The color information read by a scanner can be fed back to an image forming apparatus as accurate data by associating the color information read by the scanner with the color information read by a colorimeter. Furthermore, the basic characteristics of a colorimeter can be confirmed by obtaining the amount of reflected light and a wavelength shift amount. Therefore, when outputting an image which a user wants to print out, automatic image correction can be performed with a high productivity by reading the image with a scanner.
Meanwhile, in some case (for example, refer to Japanese Patent Published Application No. 11-243468), when a white tile is used as a reference plate, it is determined according to the level of the reflected light amount whether or not there is dirt adhering to a protective glass plate of an image sensor.
However, in accordance with this technique described in Japanese Patent Published Application No. 11-243468, it is impossible to distinguish the color information read by a colorimeter between information affected by a defect of the colorimeter itself and information affected by thermochromism.
First, wavelengths can be shifted by a defect of the colorimeter. In this case, since the colorimetric measurement result of the colorimeter is not reliable, the color information read by a scanner cannot be fed back to an image forming apparatus as accurate data by associating the color information read by the scanner with the color information read by the colorimeter.
Second, even if there is no failure in the colorimeter, wavelengths can shift under the influence of thermochromism. Specifically, a sheet discharged from an image forming apparatus has been passed through a fixing unit, and therefore heated to a high temperature. Accordingly, when an image reading apparatus read the sheet discharged from the image forming apparatus, the temperature inside the image reading apparatus is elevated because of the heat generated by the sheet. The temperature of the green tile is also elevated by the heat. Accordingly, since wavelengths shift under the influence of thermochromism, color tones are changed.
Because of this, in the case of the prior art technique as described in Japanese Patent Published Application No. 11-243468, it cannot be determined whether or not a colorimeter is in a normal condition so that accurate image correction cannot be performed.
The present invention is made in order to solve such a prior art problem and it is an object of the present invention to provide an image reading apparatus and an image forming system which can performs accurate image correction.